Dimensional measurement probe

ABSTRACT

A probe for measuring the dimensions of objects on a coordinate positioning machine such as a machine tool has a workpiece-contacting stylus  20 . This is suspended via a sensor mechanism  30 , including strain gauges  34  which provide an output when the stylus contacts a workpiece. A processor  16  processes the strain gauge outputs to produce a trigger signal. It does so in accordance with an algorithm or equation or look-up table which ensures equal sensitivity in all possible directions of approach to the workpiece in the three dimensions X, Y, Z.

The present invention relates to a probe, for measuring the dimensionsof objects, of the type used in conjunction with a machine which hascoordinate positioning such as a coordinate measurement machine, machinetool, robotic device, or the like.

Traditionally one type of dimensional measurement probe produces atrigger signal when a contact stylus touches an object, the triggersignal causing a recording of the coordinates of the coordinatepositioning machine to which the probe is attached. Sensors in the probedetect minute loadings on the stylus as the probe moves closer to theobject in order to detect contact between the stylus and the object. Forpractical reasons the contact stylus is usually an elongate stem with anenlarged end so that it can reach features of the object to bedimensionally measured.

The stem of the stylus is not completely stiff because it is relativelyslender so as to reach as many features of the object as possible.Therefore, the stylus stem will bend slightly when subjected to sideloading resulting from object contact perpendicular or oblique to theaxis stem, but it will not bend significantly when subjected to loadingcaused by contact in a direction substantially along the axis of thestem. Bending of the stylus support structure occurs also. Again thebending is greater when the stylus is loaded from the side when comparedto its bending resulting from loads along the axis of the stylus.

The result of this bending causes a trigger signal which is dependent onthe relative positions of the object surface and the axis of the stylusstem. In other words an object may appear to be a different sizedepending on the inclination of the stylus stem relative to the object'ssurface, caused predominantly by stylus bending and to a lesser extentby bending of the stylus support structure.

One way to overcome this problem is to calibrate the probe so that forexample correction factors are applied to the coordinate values when theprobe is caused to trigger due to a side loading and differentcorrection factors are applied when the probe is triggered by an“on-axis” contact. Such a calibration requires time and a calibrationartifact. It is not always possible to apply a correction factorparticularly if it is not known in which direction the contact force isbeing applied to the stylus.

Another way to overcome stylus bending is to make very sensitive thesensors which detect displacement of the stylus. Hence, little bendingof the stylus takes place before a trigger is issued. This has thedisadvantage that the probe may produce false trigger signals resultingfrom vibration of the machine or rapid acceleration or retardation ofthe probe. To overcome false triggers less sensitive sensors have to beused, although this produces a slight lag between initial contact of theobject by the stylus and the trigger signal being issued. This happensbecause there is insufficient force on the stylus initially to produce atrigger signal. Further relative movement between the stylus (and probe)and the object is required. This further relative movement is called“pretravel” and is associated with the bending of the stylus stem andthe probe mechanism mentioned above.

The stylus can be arranged such that its bending due to side loading isthe same as the displacement along the axis of the stylus stem, e.g. aresilient member positioned along the stem to imitate the sidedeflection of the stylus stem. This mechanical solution causes problemse.g. vibration of the stylus and relative manufacturing complexity.

A new approach has been provided by the inventors:

-   a measurement probe for dimensional measurement of an object by    means of contact of the object in at least two different approach    directions, comprising:-   a stylus having an object contact area and having a stem extending    substantially along an axis;-   a stylus contact determination device having a plurality of sensor    elements each having an output for detecting contact of the object    by the object contacting area of the stylus; and-   a processor for processing the output of the sensor elements,    wherein the processor processes an output of at least two of the    sensor elements to provide an object contact trigger signal caused    by a predefined displacement of the stylus object contact area,    which displacement is substantially the same in each of said    approach directions.

Thus in embodiments of the invention it may be that the contact triggersignal has issued as a result of object contact similar to that shown inFIG. 2 or 3 and in either event the trigger signal will issue only whenthe stylus contact area is displaced in its triggering direction by anamount (z in FIG. 2 or x in FIG. 3) which is equal, despite some of thatdisplacement (d) being caused by stylus stem bending. So, the positionof the stylus when a trigger signal is issued is not dependent on thedirection in which contact is made, but instead will issue when thestylus tip or object contact area has been displaced in any direction bya predetermined amount which is the same in all directions ofdisplacement.

Preferably the outputs are combined at the processor.

Preferably the outputs from the sensor elements are combined at theprocessor according to an algorithm or equation. Preferably thealgorithm or equation is

$G = {\frac{1}{\alpha}\left\{ {{\Delta\; R_{1}^{2}} + {\Delta\; R_{2}^{2}} + {\Delta\; R_{3}^{2}} + {\beta\left( {{\Delta\; R_{1}\Delta\; R_{2}} + {\Delta\; R_{2}\Delta\; R_{3}} + {\Delta\; R_{3}\Delta\; R_{1}}} \right)}} \right\}}$where the terms ΔR₁, ΔR₂ and ΔR₃ refer to the outputs of the sensorelements.

Alternatively the outputs from the sensors are processed at theprocessor so as to be compared with predetermined data.

Preferably the probe includes a probe body and preferably the processoris disposed within the probe body.

Embodiments of the invention will now be described in detail in thefollowing paragraphs with reference to the accompanying drawings, inwhich:

FIGS. 1 a and 1 b are schematic diagrams showing a measurement probeaccording to the invention, FIG. 1 b being a section on the line 1 b-1 bin FIG. 1 a;

FIGS. 2 and 3 show the measurement probe of FIG. 1 in use;

FIGS. 4 and 5 are views corresponding to FIGS. 1 a and 1 b, but showinga practical example of a measurement probe; and

FIG. 6 is a schematic circuit diagram of a processor of the probe.

In FIG. 1 a a measurement probe 10 is shown which has a body 12 attachedto the spindle of a machine tool 14. The probe has a stylus 20 having astylus tip 24 for contacting an object, in this case a workpiece 50(FIGS. 2 and 3), and an elongate stem 22 extending along axis 40. Thestem 22 is connected to the body 12 by means of a strain sensing sensor30. The sensor 30 is shown in plan in FIG. 1 b. Three fairly rigidspokes 32 each have a strain gauge 34 attached thereto for sensingstrain in each spoke, e.g. when contact is made between the workpiece 50and stylus tip 24. A processor 16 is connected to receive the outputs ofeach strain gauge.

In use the probe 10 is moved relative to the workpiece 50 in directionsX, Y and Z. Various contacts between the stylus tip 24 and the workpiece50 are made in order to determine the size of the workpiece 50. When acontact is made strain is exerted on the sensors 34. The outputs of thestrain gauges 34 are in the form of resistance change, and are processedby the processor 16 in the manner discussed below and shown in FIG. 6,to produce a trigger output signal. The trigger signal issued from theprocessor 16 can be used to record the machine tool's position in orderto determine the size of the workpiece 50.

FIG. 2 shows the probe of FIGS. 1 a and 1 b and workpiece 50. In FIG. 2the stylus 20 has been displaced longitudinally by an amount z before atrigger signal is issued by processor 16. A force will be required toimpart strain into the strain gauges 34 and thus cause the triggersignal.

FIG. 3 shows also the probe of FIGS. 1 a and 1 b and workpiece 50. InFIG. 3 the stylus 20 has been deflected laterally by distance x in orderto produce a trigger signal.

The force required to produce the trigger signal when contact is at theside of the tip (as in FIG. 3) will cause the stylus to bend.Conventionally, z<x because the stylus is much stiffer in thelongitudinal direction than laterally. If z<x then the apparent measuredsize of workpiece 50 will be different in the X (and Y) directions thanthe apparent measured size in the Z direction.

However, in this embodiment the trigger signal is produced when z isapproximately equal to x. This is achieved by combining the outputs fromthe three strain gauges in the processor 16, to form a gauge output G asfollows:

$G = {\frac{1}{\alpha}\left\{ {{\Delta\; R_{1}^{2}} + {\Delta\; R_{2}^{2}} + {\Delta\; R_{3}^{2}} + {\beta\left( {{\Delta\; R_{1}\Delta\; R_{2}} + {\Delta\; R_{2}\Delta\; R_{3}} + {\Delta\; R_{3}\Delta\; R_{1}}} \right)}} \right\}}$where α is a normalisation/scaling factor;the terms ΔR₁, ΔR₂ and ΔR₃ refer to the change in resistance of thethree strain gauges 34; and β is a further factor dependent on thestrain characteristics of the structure of the sensor 30 and the stylus20 strain characteristics.

The principle behind the equation above is that the output of each ofthe gauges may be decomposed into a component arising purely from thecomponent of the contact force acting along the axis 40, and a componentarising purely from the component of the contact force actingperpendicular to the axis. These components of the gauge output may thenbe combined and manipulated to give different sensitivities in X and Ydirections compared with the sensitivity in the Z direction. The resultof this manipulation is that the object detection caused by contact inthe Z direction (as shown in FIG. 2) can be desensitised so that morecontact force is required to produce a trigger signal than the forcerequired to produce a trigger signal when the contact is of the typeshown in FIG. 3. Thus the distance z and x can be made equal orsubstantially or approximately equal, despite the x deflection having acomponent (d) resulting from bending of the stylus stem 22. The same butrelative effect also occurs for all intermediate directions between xand z. This makes the contact signal trigger output independent of thedirection of contact. A predefined movement of the stylus tip in anydirection relative to the stylus thus produces a trigger signal output.

The processor 16 preferably comprises a combination of analoguecircuitry. This is configured in a known manner with a module 16A whichcalculates G in real time in accordance with the above equation. It alsocomprises a module 16B which is configured to produce the trigger outputwhen the value of G exceeds a predetermined threshold value. Thiscircuitry of the processor 16 may be implemented as anapplication-specific integrated circuit (ASIC).

Alternatively, however, the processor 16 can be implemented in a digitalform, with a suitably programmed digital arithmetic unit. The outputs ofthe strain gauges can be converted to digital signals usinganalogue-to-digital converters. The processor 16 can include programmodules corresponding to the modules 16A, 16B. The equation presentedabove can be maintained in non-volatile memory in the processor 16,which can then constantly monitor and process the strain gauge outputs.

Alternatively the processor 16 can compare the gauge outputs with a datatable, in real time. A trigger signal output will be produced if thecomparison indicates that the outputs of the strain gauges areindicating a triggered condition. In practice this can be achieved byusing for example an analogue to digital converter for each straingauge, the outputs of which form an address bus connected to the datatable memory. An output from the memory of “0” (no trigger) or “1”(trigger) can be used.

The values of α and β are normally held constant during a givenmeasurement. However, small variations may be applied to them, e.g. tooffer lower vibration sensitivity, or used to detect the probingdirection (Z versus X, Y). Changes in stylus length can be accommodatedby changing the value of β or if a data table is used, having differenttables for different styli. The data for the table can be pre-programmedor “taught” by exercising the stylus and teaching the probe to triggerat certain deflections. Gaps in data can then be infilled using a meshalgorithm. The processor 16 may be located away from the probe e.g. inan interface between the probe and the machine to which it is attached.

The equation presented above assumes the gauges 34 are symmetric aboutthe axis Z, but with modification a similar equation can be used forother configurations of sensor 30, having two or more sensing elements.

The sensor 30 is shown having strain gauges extending radially of axisZ. However, the sensor need not use strain gauges and the sensingelements need not extend radially. For example the sensing elementscould be displacement sensors like LVDTs, linear encoders or capacitancesensors. Their arrangement could be axial or any position which allowssensing of stylus contact.

The mechanical arrangement of a practical probe is shown in FIGS. 4 and5, by way of example. It will be appreciated that other arrangements arepossible. For clarity the view of the probe in FIG. 4 is a part-sectionin the sectional plane denoted 4-4 in FIG. 5. This plane is notcompletely flat but includes two planes at 120° to each other.

A dimensional measurement probe 110 is shown which is attachable to amachine 115 via a boss 112. The machine is typically one which candetermine the coordinates of the probe e.g. in xy and z planes.

The probe has a stylus 114 including a tip 116 for contact with aworkpiece or other artifact 150 to be measured. The probe is moved bythe machine relative to the artifact 150 and contact of the stylus tipwith the artifact 150 is sensed by the mechanism within the probe 110.The probe produces a trigger signal which is sent to the machine inorder to determine the probe's coordinates. In this way the coordinatesof the surface of the artifact can be obtained.

The probe 110 includes a main body 118, a circuit board 120, a springcage 122, a compression spring 124, upper member 126 of stylus 114 and astrain sensing element 130. The circuit board 120 contains the processor16 discussed above in connection with FIGS. 1-3 and FIG. 6. Otherarrangements are of course possible, e.g. a flexible circuit board maybe wrapped cylindrically around the mechanical components of the probe.

In operation force is exerted on the stylus tip in the x, y or zdirections or combinations of these directions. The force causes flexingof the radially extending arms 132 of the sensor element 130 relative tothe body 118 to which the sensor element is fixed at central portion137. Excessive force on the stylus in the x or y directions, or pullingthe stylus in the z direction away from the probe body, will result inclosing of the gap 128 between the sensor element 130 and the body 118.Thus excessive strain on the sensor element 130 cannot take place.Further force on the stylus causes the compression of spring 124resulting in the disconnection between the stylus upper member or stylusholder 126 and the sensor element 130 against the force of thecompression spring 124. Removal of the further excessive force allowsthe stylus to reseat against the sensor element 130. The contact betweenthis upper member 126 and the sensor element 130 is in the form of akinematic location having a total of six points of contact. In thisinstance the kinematic location is formed from three balls 131 on thesensor, each one nesting between a pair of rollers 127 on the member126. Thus advantageously, reseating is possible into a repeatable restposition if excessive force is exerted on the stylus. However, anon-kinematic location is also possible.

FIG. 5 shows the sensor element 130 in more detail. The element 130 isproduced as one piece, e.g. of machined metal. Each of the three balls131 affixed to the surface of the element 130 has in use force exertedon them. When the stylus tip 116 contacts the workpiece the forceexerted on the balls is altered. This in turn causes strain to beinduced in radial arms 132. A semiconductor strain gauge 133 is securedto each of the arms 132. Each strain gauge provides a change in outputif the strain in the arm is altered. Thus stylus contact with article150 can be detected.

Three radially extending arms 132 are shown, although other numbers ofarms may be employed. Further or alternative details of the probe may beas shown in International Patent Application No. PCT/GB2006/001095,which is incorporated herein by reference.

1. A measurement probe for dimensional measurement of an object,comprising: a stylus having an object contacting area and having a stemextending along a first axis; a stylus contact determination devicehaving a plurality of sensor elements, each sensor element having anoutput for detecting physical contact of the object by the objectcontacting area of the stylus; and a processor for processing theoutputs of the sensor elements; wherein the outputs of the sensorelements are combined in the processor such that the sensitivity when anobject is approached along an approach direction parallel to the firstaxis is different compared with the sensitivity when an object isapproached along an approach direction perpendicular to the first axis,whereby the processor provides an object contact trigger signal causedby a predefined displacement of the stylus object contacting area, thepredefined displacement being approximately independent of the approachdirection between the measurement probe and the object, and theprocessor combines the outputs of the sensor elements to form a gaugeoutput (G) according to the algorithm or equation$G = {\frac{1}{\alpha}\left\{ {{\Delta\; R_{1}^{2}} + {\Delta\; R_{2}^{2}} + {\Delta\; R_{3}^{2}} + {\beta\left( {{\Delta\; R_{1}\Delta\; R_{2}} + {\Delta\; R_{2}\Delta\; R_{3}} + {\Delta\; R_{3}\Delta\; R_{1}}} \right)}} \right\}}$where the terms ΔR₁, ΔR₂ and ΔR₃ refer to the outputs of the sensorelements and α and β are scaling factors.
 2. A measurement probeaccording to claim 1, wherein the processor compares the outputs of thesensors with predetermined data in a look-up table.
 3. A measurementprobe according to claim 1, wherein each sensor element comprises astrain gauge.
 4. A measurement probe according to claim 1 comprising aprobe body, wherein the processor is disposed within the probe body. 5.A measurement probe according to claim 1, wherein the stylus contactdetermination device comprises three sensor elements.
 6. A measurementprobe according to claim 1, wherein the sensor elements are symmetricabout the first axis.
 7. A measurement probe according to claim 1,wherein the sensor elements extend radially about the first axis.
 8. Ameasurement probe according to claim 1, wherein the processor comprisesanalogue circuitry.
 9. A measurement probe according to claim 1, whereinthe processor comprises a digital unit.
 10. A measurement probeaccording to claim 1, wherein the stylus is stiffer in a longitudinaldirection along the first axis than in a lateral direction.
 11. Ameasurement probe according to claim 1, wherein the stylus can bend in adirection perpendicular to the first axis.